Process for production of alkali metal sulphide and hydrosulphide of high purity and concentration



Allg 419, 1941- I J. sysc'oNcE Erm. 24,252,867

HYDROSULPH IDE l PROCESS FOR PRODUCTION OF ALKALI METAL SULPHIDE `AND OF HIGH -PURITY AND CONCENTRATION V Filed April 2l, 1959 1NVENT0R. A ma( a MW W Patented Aug. 19, 1941 rROCEss 'FOR PRODUCTION 'OF ALKALI METAL SULPHIDE AND HYDROSULPHIDE OF HIGH PURITY AND CONCENTRATION .lames S. Sconce and Charles F. Berlingho, Ni-

agata Falls, N. Y., assignors to Hooker Electrochemical Company, New York, N. Y., a corpo ration of New York Application April 21, 1939, Serial No. 269,186

12 Claims.

`Sodium hydrosulphide and sulphide can be produced by reaction of hydrogen sulphide with caustic soda as follows:

solution turns from a reddishbrown to a dark green, due to the contaminating metals having been converted to their dark sulphides, which in a yellowsolution appear green. The length of this period of further treatment depends upon l the degree of contamination of the solution, but Reaction 2 is obviously merely a continuation may amount to as much as twenty-live per cent of Reaction l. of the whole time of treatment. The resulting lSince it is dl'fcult t0 react hydrogen sulphide sulphides are colloidal and when first formed too With nhydrOuSCauStC SOda, the CauStC SOCa. neto be ltered out. .If the contamination is for the purpose of these reactions, may be understood to be in solution. It is possible to make andusre, solutions of caustic soda of concentrationsup to '78 per cent, but for practical reasons asolution having a concentration of about '70 per cent is preferred. This corresponds to NaOI-I.95

HzO.f.It Will also be noted that water is a product of the reaction. For practical purposes therefore, these reactions should be written:

gspectively, which crystallize upon cooling.

Reactionsvl, 2, 3 and 4 are strongly exothermic slight and the treatment has been relatively brief, it may be necessary to allow the solution to stand from a few minutes to an hour ormore to allow the precipitate time in which to coagulate suiiiciently to be removable By this special treatment, therefore, We are enabled toremove a large proportion of the contaminants, either by ltration or by allowing the product to settle and crystallize; in which latter case the impurities will be found to be concentrated in the bottom portion.

It is also desirable and an object of our invention to reduce the water content ofthese products, not only to save freight in shipment, but

l also because sodium hydrosulphide of 60 to 62 angelsnigflelslrntgirilng Water into the reaction per ce2; (enaCe-mtne blyrrnsgutlleilntmttg the -caustic soda introduces numerous soluble im- Isnurtllds -Of reducing the Water Content, `on@ of lrlt. cngglglgaagsr otalsg which involves elrf'aporation1 anitlhe other; o; 1;' .de no. O-rexam e,1 e ro uc o are not necessarily in the form of their soluble vllcclior 3s, be reacted Withpmore hydogen 5u1 salts, but in the minute quantities in which they phide, We Shall have the following result:`

occur` may be actually in solution or colloidal suspension in metallic form. All of these metals, NazS-3.9l-I2O-l-H2S2(NaSI-I- 1.95H2O) (5) even 'in suchquantities, are objectionable in the ,35 processes for which these products are intended, This `0f COUTS@ iS merely a @Onl'flluaton 0f Re- ,but this is particularly true of the iron, which 'actionY 3 and the product is the` 61.5 per cent isnot' `only catalytic but liable to cause dis- `product of Reaction 4. Since this productv is adoration. 'It is very desirable, and one of the easily melted', it may be reacted Wlth Substantialobjects of our invention, therefore., to reduce the yp 1y `mhydIOllS flake@ Caustic SOf. a1S OHOWSI quantity and number of these contaminants.

`It is known that these metals are precipitated 2 NaSH`195`H2+2NaOH as their sulphides from relatively weak solutions 2(Na2S'295H20) (6) of sodium sulphide (e. g. to 32 percent) produced by passing hydrogen sulphide through rel- .Thls 1s a2 95 per CEM podctfa Cmparq atively weak solutions of caustic soda (e. g. 25 Wlth the 5 per Cep pro .c 1h tea; n percent). When the caustic soda is of the conrilgegver the quantlty 1s twlce a o eac' centration essential for our purpose (e. g. 70 percent) and the metallic contaminants such as Slmla'rtyil 1f the hploduct ofl lction hbl are ordinarily brought in with the caustic soda eac e?) n-H more y-lfgen su p l e We S a no precipitation of metals whatever occurs in met e 0 Owing resu the resulting solution of sodium sulphide. How- 2(Na2S.2.95H2O) +2H2S 4(N9,SH.1 475H20) (7) ever, we have discovered that when the caustic` 4 soda has been fully converted to sodium hydro- This is a 67.9 per cent product, as compared sulphide by treatment with hydrogen sulphide in with the 61.5 per cent product of Reaction 3, and accordance with Reaction 4, so that no sodium. aga-ifi the quanttyis'dmlbled.

sulphide remains, these 4impurities may be, to a Hence, by alternating Reactions 6 and 7 We can large extent, precipitated. For` this purpose it go on building up the quantity of product, and is necessary to continue the treatment with hyalso, within practical limits, its concentration. drogen sulphide for a further period until the When' a concentration of about 65 per cent of sodium sulphide or about 70 per cent of sodium hydrosulphide had been reached, it would beV necessary to begin adding water or using a certain percentage of the caustic soda in the form of an Y aqueous solution, in order to `continuato carry 011.,.5

ablefor economic reasons," as well `as for pracl tical reasons, to use caustic soda in the form of its solutions, rather than solid fused caustic sodat0 as large an extent as compatible with production of sodium sulphide or hydrosulphide of the desired degree ofconcentration. Obviously oneway l5 of servingthis purpose would be by concentrating the vproduct inr anlevaporator, VIt is not practicable to.. concentratev sodiumv sulphide or hydrosulphidein steel, as the, evaporator lasts only a ,few days.. It ispossible todo soin cast iron. but

the .product-becomes badly contaminated; Howeverwe. have discovered' that sodium hydrosulphide-.can be concentrated, substantially without .decompositionlor contamination, in an evapo- Y rator. of. chrome-nickel steel of. they special f25-20, alloyf which contains 25v per cent chromiumand v20.-!pe'r centnickel. (ordinary. 18-8 alloybeing unsuitable), and. that it is cheaper to doiso thanltocencentratec austic vsoda beyond 70 pencent.,

Qur preferred process. therefore begins with Reaction-1, which, however, is used only at the startof thev` process,l for the following reasons: ,Ifk vthe foregoing freactions/arecarried on in a reactor of the iron-nickel-cobalt group, serious Y contamination resultsand the reactor does not lasttlong. 'Ifh'isisalso trucoff the non-ferrous metals. `glv,e1ir1.c1;` that in the. case oiiReaotions 3 and, Vtrwnicn. involve.. fairly longf contact of the reactor .wallsfwithstrong caustic soda while the 40 Has' is. being.bubbled. in, these reactions can be practicedin a.25.-20v chrome -nickel 'steel reactor, butLthisjmaterial is Very expensive for this purpose. However, `weflndthat reactions which do notlinvolve. strong concentrations of rcaustic soda 45 or whichtake place quickly,.such asy Reactions 5 and 6, can be carried on in a reactor of a nonferrous Ymetal without contamination ofthe product or attack upon the reactor. One of the cheap- 30 purities.

Theproductof this reaction is of course the 52.7 per cent product of Equation'3, although produced by a different reaction. This is then reacted with more hydrogen sulphide in accordance withReaction 5, thus replenishing the hyldrosulphide that was withdrawn. Thus we alternate Reactions. 8 and 5, removing preferably about two-thirds of thehydrosulphide, by volurne,v after eachalternation. The reactor is therefore never less than one-third filled with liquid, which is always largely hydrosulphide, even after lReaction 8; and since ReactionV 8 is practicallyV instantaneous, the reactor is never subjected to strong concentrations of caustic soda. In this way we are able to use a lead lined reactor and avoidv increasing the iron contaminationv beyond that due to the iron already inthe caustic soda.

Just as the contaminants may berthrown out of solution following'Reaotion 4, we nd that they may be similarly thrown out following Reaction 5. In other words, after the metallic contaminants have, been converted to their sulphides by adequate treatment with HzS, they cannot remain in solution to any considerable extent in sodium hydrosulphide solutions which are free from so.- dium. sulphide. The next step therefore is to filter the sodiuml hydrosulphide solution withdrawn from the lead lined reactor, to remove these im- It is of considerable interest tonote that the filter cake is quite pyrophoric andthe filter medium, which is preferably of. paper, is liable to burst into flame, even while still damp. This is probably due to the presence of colloidal iron sulphide, which is known to be highly. pyrophoric. The Vfiltered'hydrosulphide isthen concentrated to a` 70 per cent solution, represented by the formula NaSHfLBZHzOAn thechromenickel steel evaporator above specified. If! this is the product wanted, it is cooled,` iiakedlandpacked forfthe market. If sodium sulphide isthe product wanted, itis produced by the. Vfollowing reaction:

This is a per centy product. It 'is' then cooled, flake'dland packaged for themarket.` j t Example l Crude commercial sodium sulphide, prepared inaccordancev with Reaction` 3 was converted' in est?. and `best metals of this,Y category is lead, 50 aflead lined reactor to the. hydrosulphide` in ac- Therefore; after starting the-process with ReactionftQwevv-ithdraw from the reactor only a portion-.oft the: hydrosulphide thereby produced and reactv the-remainder, or asrmuch'of: it as may be cordance with Reaction 5. YThe hydrosulphide was treated with an excess of H'zS' until the color V.turned toV a dark green, and passed through a filter press. The filtrate was concentrated inpa necessary to replace the Ywithdrawn portion, in a 55,y chrome-nickelsteel evaporator and reconverted lead lined reactor, with percentcaustic soda solution, in accordance with the following equation:

to twice the original quantity of sodium sulphide inA accordance with Reaction 9. TheA analyses of the original, intermediate and, nal' products were Nasi-:- Lesmo-entron- .snzoenazsaorno (s) "*nas follows:-

(11) (b) (C) (d) (6)- (f) V (E).

' Brod- Cu-d" Cmd@ Filtrate uct of product produc-t Flller' Filtrate (d) after NaOH (6)' and of of Ca e from b concen- (lieke) (f) by reac reacl'rom b tration reac tion 3 tion 5 tion 9 12. 26 0. 26 0 24 2. 97 1. 5, 960. U 90 12. 0 6, Undeter. 50 40 90 Undeter 30 30 l. 40 Undeter Tr. Tr., Tr. T Undeter. 3. 0 5. 0 73. 9 15.

Comparing columns (a) to (d) of this example, we note that the impurities of the product of Reaction 3 were carried over to Reaction 5 Where they were largely precipitated. These impurities .were removed by the filtration step, as shown by column (d), appearing in the filter cake, as shown by column (c). It will also be noted that there was only a negligible increase in impurities during the evaporation step, as shown by column (e). This column represents finished product, if sodium hydrosulphide is wanted. The result of converting the hydrosulphide to the sulphide `in accordance with Reaction 9 is shown by columns (f) and (y). The impurities in the caustic soda are shown in column (f) and these of course carry over into the final product. Nevertheless, comparison of column (g) with column (a) shows a substantial increase in concentration and at the same time a reduction of the contaminants, especially of the iron.

Referring to the flow sheet:

1 is ajacketed lead lined reactor, in which initial Reaction 4 and subsequent alternating Reactions 5 and 8 take place. Seventy per cent caustic soda solution at 100 C., from a source not shown, is admitted through pipe 3, controlled by valve 2, alternately with hydrogen sulphide, from a source not shown, which is admitted through pipe 5, controlled by valve 4. The hydrogen sulphide is conducted to a point near the bottom of reactor I and bubbled through the body of liquid in the reactor. The treatment with hydrogen sulphide is continued until the solution turns a dark green, due to a black precipitate in the yellow solution, anduntil the resulting precipitate has had time to ccagulate. Reaction 5 is decidedly exothermic. Reaction 8 is less so. Water is circulated through jacket 28 at 90 C. and this serves to cool `Reaction 5 and warm Reaction 8. These reactions therefore take place at between 90 and 110' C. After each repetition of Reaction 5 a portion of the resulting sodium hydrosulphide is drawn ofi' from reactor I through pipe 6 by opening valve "I, and, while still hot, forced by pump 8 through pipe 9 and lter press II). From the filter press the hydrosulphide solution flows by gravity through pipe II to -20 chrome-nickel steel evaporator I2. The filter cake from the filter press is dischargedto storage I3 through chute I4.

Evaporator I2 is heated by .iets of flame from burner I5. The concentrated hydrosulphide is raised `by submerged pump I6, driven by motor I1, through pipe I8 to liquid product storage I9, which is of 25-20 chrome-nickel steel. If sodium hydrosulphide is the product wanted, the solution is fed from storage I9 through pipe 20, controlled by Valve 2|, directly to flaker 22 whence the flaked product is delivered through chute 23 to container 24. If sodium sulphide is the product wanted, flake caustic soda from storage 25 is admitted vthrough chute 26, which is controlled by gate 21, to liquid storage I9. The flake caustic soda reacts instantly with the hydrosulphide in liquid storage I9, converting it to the sulphide, which is flaked and packed as before.

When concentrated to a 65 per cent solution or higher and cooled to 100 C., sodium sulphide crystallizes to its monohydrate, which is 81.2 per cent sodium sulphite. The crystals, if separated from their mother liquor, will, of course, be found to have been greatly purified. Theoretically, they should be extremely pure, but such crystals are always wet with liquor, in which the contaminants are concentrated, so that the degree of purication depends to considerable extent upon the thoroughness with which the crystals are separated from their liquor. Any of the wellknown processes, such as centrifuging, washing, remelting a portion and recentrifuging, may be employed for this purpose. 1

When evaporated to substantially a per cent solution and cooled, sodium hydrosulphide crystallizes to `a mixture of NaSH-1HzO and NaSH-ZHzO which is about 70 per cent sodium hydrosulphide. This is a white crystalline product melting at 50 to 55 C.

When evaporated to about per cent solution, `the solution of sodium hydrosulphide suddenly solidies without cooling to sodium hydrosulphide monohydrate, which is 75.6 per cent sodium hydrosulphide.

Sodium sulphide is used as a desulphuring agent in the viscose rayon industry. Sodium hydrosulphide may be used for the same purpose by rst converting it to the sulphide. Metallic impurities, and in particular iron, are very objec tionable in this industry as they tendto discolor and weaken the product. Our sodium sulphide and hydrosulphide of high purity are, therefore very valuable in the viscose rayon industry.

Sodiumv sulphide and hydrosulphide are also used for dehairing hides. The hides are irnmersed in a dilute solution of sodium sulphide or hydrosulphide in milk of lime for several days. The hair is softened and in some treatments actually dissolved. For this purpose the hydrosulphide is preferred, as it contains twice as much sulphur in proportion to sodium. A further advantage of the use of hydrosulphide results from its buffering action in maintaining a lower pH during the dehairing process. For example, commercial sodium sulphide may contain several per cent of sodium carbonate. In presence of a warm milk of lime suspension this will be converted to sodium hydroxide. This sodium hydroxide will attack the hide, causing excessive and irregular swelling of the different types of proteins. This weakens the leather and causes a rough, grainy appearance. When the hydrosulphide is used, even if it contains sodium carbonate, there will be no harmful effect, as the resulting sodium hydroxide .is neutralized, as rapidly as formed, to sodium sulphide.

In the leather industry likewise metallic impurities are objectionable. Iron, for instance, forms insoluble deep colored tannates which stain the hides and the presence of iron and copper salts catalyzes reactions resulting in injury to the finished leather and development of objectionable odors. Our'sodiiun sulphide and hydrosulphide oi high purity are, therefore, very valuable for dehairing of hides.

We are aware that sodium hydrosulphide produced from sodium sulphide made by reduction of sodium sulphate has been filteredto remove gross impurities such as relatively coarse particles of carbon and ash introduced during the reduction; but such particles are of enormous size compared with the colloidal precipitate of metal sulphides with which we are concerned; moreover, having never been in solution they cannot be said to have been thrown out of solution or precipitated.

We claim as our invention:

1. The process for production of alkali metal sulphide of relatively high concentration and low iron content which comprises treating a substantially 70l 150.78l per cent solution of the hydra Woff 4,aid alkali metal, of ordinary comgrade and relatively high contamination byhmetals, withfhydrog'en sulphide, until the hydro'xide'hasbeen completely converted to the corresponding` alkali metal" hydrosulphide Vand the colory of the solution has changed from reddislrhrown to dark green and hydrogen sulphide begun to bubble through the solution; continuing saidtreatmentl for a prolonged further period until the contaminating metalsin the hydrosulphide have been Vsubstantially completely converted to their sulphidesf and the latter changed from a colloidal form, to a readily lterable precipitate; removing said precipitate; and converting the resulting puried hydrosulphide to sulphidev by reaction with more hydroxide of said alkali metal, but of relatively high concentration and low contamination by metals.

2. The process forY production of alkali metal sulphide of relatively high concentration and low iron content which comprises treating a substantially 70 to 78 perV cent' solution of the hydroxide of said alkali metal, of Yordinary commercial grade' and relatively high contamination by metals, 'with hydrogen sulphide, 'untilthe hydroXide has been completeley converted to the corresponding alkali metal hydrosulphide and the color of the solution has changed from reddish brown to dark green and hydrogen sulphide begun 'to'lbubble through the solution; continuing said treatment for a prolonged further period until the lcontaminating metals in the hydrosulphidel'have been substantially completely converted to their sulphides and the latter changed from colloidal to readily lterable form; ltering out 'the contaminants from said solution; and converting the resulting puried hydrosulphide to sulphide by reaction with more hydroxide of said alkali'metal, but or relatively high concentration and low contamination by metals.

37.*The process for production of alkali metal hydrosulpljiidel of relatively high concentration and lpw iron content which comprises treating a substantially 'loto '78' percent solution of the hydroizide of said alkali metal, of ordinary commercial gradeand relatively high contamination by metals, with hydrogen sulphide, until the hydroxidel hasbeen completelynconverted to the corresponding alkali metal hydrosulphide and thev color ofthe solution has changed from reddish bro-wn to dark green and hydrogen sulphide begun to bubble through the solution; continuing said treatment for a prolonged further period unti the contaminatingdmetals in the hydrosulplaide have beensgibstantially completely converted to their sulphides and the latter changed from colloidal form to a readily flterable precipitate; allowing the solution to cool, settle and crystallize; and separating the relatively pure upperpprtion from the remainder.

n 4. V*The process for production of alkali metal hyolrosulphi-de of relatively high concentration and low iron content Awhich comprises treating a substantially T0. to '7,8 per cent solution ,of the hydroxide of said alkali metal, of ordinary commercial grade and relatively high contamination by metals, with hydrogen sulphide, until the hydroxide has been completely converted to the corresponding alkali metal hydrosulphide and the color of the solution has-changed from reddish brown to dark green and hydrogen sulphide Y begun to bubble through the solution; continuing said treatment for a prolonged further period from a colloidal form to a readily iilterable pr@-r cipitate; 4and removing said precipitate.

5. The process for production of alkali metal hydrosulphidel of relatively high concentration and low iron content which comprises treating a solutionloi the hydroxide of. said alkali'metal, of ordinary commercial concentration and grade and relatively high Vin contamination by metals, with hydrogen sulphide, until. the hydroxidepha's been completely converted to the corresponding alkali metalihydrosulphide' andthe c olor of .the solution has changed from reddish brown todark green and hydrogen sulphide begun to bubble through the solution; continuing said treatment for a prolonged further period until the contaminating metals in the hydrosulphide have been substantially completelyfconverted to their sulphides and the latter changed from a colloidal form to a readily lterable precipitate; removing said precipitate; concentrating the solution; and allowing the solution to cool'and'solidiiy.

6. The process for production of alkali metal hydrosulphide of relatively high concentration and low iron content which lcomprises treating a solution of the hydroxide-oi said alkali'metaLgof ordinary commercial concentration and grade and relatively high in contamination' by metals, with hydrogen sulphide, until the hydroxidehas been co-mpletely converted to thejcorresponding alkali metal hydrosulphide and the color of the solution has .changed from reddish brown'. to dark green and hydrogen sulphide begunv to bubble through the solution; continuing said treatmentV for a prolonged further period until the-contaminating metals in the hydrosulphide have been substantially completely converted to their sulphides and the latter changed from a colloidal form to a readily lterable precipitate; removing said precipitate; concentrating the solution by evaporation; and allowing the solution to' cool and solidify.

'7. The process for production of alkali'metal hydrosulphide of relatively high concentration and low iron content which comprises treating a solution of the hydroxide of said alkali metal', of ordinary commercial concentration and grade and relativelyv high in contamination by metals, with hydrogen sulphide, until the hydroxide has been completely converted to the corresponding alkali metal hydrosulphide and the color of the solution has changed from reddish brown t dark green andv hydrogen sulphide begun to bubble through the solution; continuing said treatment for a prolonged further period, until the Vcontaminating metals in the hydrosulphide'have been substantially completely converted to their sulphides and the latter changed from l a colloidal Aform to a readily lterable precipitate; removing said` precipitate; concentrating the solution by evaporation to lsubstantially 70 per cent; and allowing the solution to coolV and solidify.

8. The process for production of alkali metal sulphide of relatively high concentration -and low iron content whichlcomprifses treating substantially TQ to 7,8 per cent solution of the hydroxide of said alkali metal, of Yordinary commercial grade and relatively high Ycontainination metals, with hydrogen sulphide, until the hydroxide has beencompletely converted to the, corresponding alkali metal hydrosulphide andthe color of the solution has changed from reddish brown to dark green and hydrogen sulphide begun to bubble through the solution; continuing said treatment for a prolonged further period until the contaminating metals in the hydrosulphide have been substantially completely converted to their sulphides and the latter changed from a colloidal form to a readily filterable precipitate; removing said precipitate; converting the resulting puried hydrosulphide to the corresponding sulphide by reaction with more hydroxide of said alkali metal, but in anhydrous form and relatively low in contamination by metals; and allowing the resulting alkali metal sulphide to cool and solidify.

9. The process for production of alkali metal sulphide of relatively high concentration and low iron content which comprises making up a solution of the hydrosulphide by treating a solution of the hydroxide oi said alkali metal, of ordinary commercial concentration and grade and relatively high in contamination by metals, with hydrogen sulphide, until the hydroxide has been completely converted to the corresponding alkali metal hydrosulphide and the color of the solution has changed from a reddish brown to a dark green and the hydrogen sulphide begun to bubble through the solution; continuing said treatment for a prolonged further period until the contaminating metals present have been substantially completely converted to their sulphides and the latter changed from a colloidal form to a readily lterable precipitate; removing said precipitate; concentrating the solution by evaporation; converting the resulting puried hydrosulphide to the corresponding sulphide by reaction with more hydroxide of said alkali metal, but in anhydrous form and relatively low in contamination by metals; and allowing the resulting alkali metal sulphide to cool and solidify.

l0. The process for production of alkali metal sulphide of relatively high concentration and low iron content which comprises treating a solution f the hydroxide of said alkali metal, of ordinary commercial concentration and grade and relatively high in contamination by metals, with hydrogen sulphide, until the hydroxide has been completely converted to the corresponding alkali metal hydrosulphide and the color of the solution has changed from a reddish brown to a dark green and the hydrogen sulphide begun to bubble through the solution; continuing said treatment for a prolonged further period until the contaminating metals in said hydrosulphide have been substantially completely converted to their sulphides and the latter changed from a colloidal form to a readily iilterable precipitate; removing said precipitate; concentrating the solution by evaporation; converting the resulting puried hydrosulphide to the corresponding sulphide by reaction with more hydroxide of said alkali metal, but in anhydrous form and relatively lowin contamination by metals; and allowing the resulting alkali metal sulphide to cool and solidify.

1l. The process for production of alkali metal sulphide of relatively high concentration and low iron content which comprises treating a substantially 7) to 78 per cent solution of the hydroxide of said alkali metal, of ordinary commercial grade and relatively high contamination by metals, with hydrogen sulphide, until the hydroxide has been completely converted to the corresponding alkali metal hydrosulphide and the color of the solution has changed from a reddish brown to a dark green and the hydrogen sulphide begun to bubble through the solution; continuing said treatment for aprolonged further period until the contaminating metals in the hydrosulphide have been substantially completely converted to their sulphides and the latter changed from a colloidal form to a readily lterable precipitate; removing said precipitate; converting the resulting purified hydrosulphide to the corresponding sulphide by reaction with more hydroxide of said alkali metal, but in anhydrous form and relatively low in contamination by metals; crystallizing the resulting sulphide out from said solution .and separating the crystalline product from said solution.

12. The process for production of alkali metal hydrosulphide of relatively high concentration and low iron content which comprises: (a) treating a solution of the hydroxide of said alkali metal, of ordinary commercial concentration and grade and relatively high in contamination by metals, with hydrogen sulphide, until the hydroxide has been converted to the corresponding alkali metal sulphide; (7J) treating the alkali metal sulphide with hydrogen sulphide until it has been completely converted to the corresponding alkali metal hydrosulphide and the color of the solution has changed from reddish brown to dark green and hydrogen sulphide has begun to bubble through the solution; (c) continuing said treatment for a prolonged further period until the contaminating metals in the hydrosulphide have been completely converted to their sulphides and the latter changed from a colloidal form to a readily lterable precipitate; (d) removing a part only of the resulting puriiied hydrosulphide together with at least a corresponding quantity of` precipitate; (e) converting a par-t only of the remaining hydrosulphide, equal in quantity to substantially half the quantity withdrawn, to the corresponding alkali metal sulphide, by reaction with more solution of said hydroxide; 'continuing the process by repeating steps (l2), (fc), (d) and (e) in rotation; and removing the precipitate from the withdrawn hydrosulphide.

JAMES S. SCONCE.

CHARLES F. BERLINGHOFF. 

